1. Field of the Invention
The present invention relates to an optical head and a disk unit, and more particularly, it relates to an optical head and a disk unit that attain an optical head having a small thickness and a light weight to enable a disk unit, such as an optical disk, a magnetic disk and a magneto-optical disk, realizing a small size and a high recording density.
2. Description of the Related Art
Optical disks including magneto-optical disks are demanded to have a high density and a large capacity. According to the road map organized by Optoelectronic Industry and Technology Development Association (Japan), it is requested that the recording density attains 1 Tbits/(inch)2 in 2010 (Road Map in information recording field by Optoelectronic Industry and Technology Development Association (Japan), March of 1998).
As a major candidate therefor, magneto-optical recording using near-field light from a solid immersion lens (SIL) can be exemplified. In this technique, recording is carried out by using near-field light (or evanescent light) leaked from a light spot on a light converging surface of an SIL having a semispherical shape or a super semispherical shape (B. D. Terris, et al., Appl. Phys. Lett., vol. 65(4), p. 388 (1994)).
In this recording technique, the distance between the SIL and the recording surface is necessarily 100 nm or less as calculated by K. Otaki (K. Otaki, Technical Research Report of Institute of Electronics, Information and Communication Engineers, CPM98–110, p. 30 (1998)), and it is necessary that the optical head is run in a flying state at an altitude around 100 nm or less. Accordingly, the disk is necessarily of a non-removable type to avoid invasion of dusts, and therefore, it is necessary that the height of the optical head is as low as that that of a magnetic head used in a fixed magnetic disk unit. Magnetic heads having a height of about 0.5 mm or less (pico heads) are being brought into the market as the most advanced products. Particularly, in the case of a so-called Winchester type disk unit having plural disks stacked, reduction of the distances between the disks is important for increasing the volume density, and there arises a demand for reduction of the height of the head inserted between the disks. In the case of a removable disk, the size reduction and the weight reduction of an optical head are important for such an operation as high-speed tracking and high speed seeking.
Examples of the conventional optical head using an SIL include those disclosed in B. D. Terris, et al., Appl. Phys. Lett., vol. 65(4), p. 388 (1994).
FIG. 12 is a lateral cross sectional view showing the conventional optical head. The optical head 1 has a semiconductor laser 2 emitting a laser beam 3a, a collimator lens 4 shaping the output beam 3a from the semiconductor laser 2 into a parallel beam 3b, a folding mirror 6 reflecting the parallel beam 3b from the collimator lens 4 at a substantially right angle, an objective lens 5 converging the parallel beam 3b reflected by the folding mirror 6, an SIL 7 forming a light spot 9a on a light converging surface 7c with a focused beam 3c incident from the objective lens 5, and a flying slider 8 running in a flying state and supporting at least the semiconductor laser 2, the collimator lens 4, the folding mirror 6, the objective lens 5 and the SIL 7.
In the optical head 1, the semiconductor laser 2 is arranged to make the optical axis 2a of the emitted laser beam 3a parallel to the optical disk 10, and the laser beam 3a is collimated with the collimator lens 4 to form the parallel beam 3b, which is then perpendicularly reflected with the folding mirror 6 to make incident on the objective lens 5. Therefore, the height of the optical head can be decreased in comparison to such a constitution that the semiconductor laser 2 as a light source is arranged on the optical axis 7a of the SIL 7.
Instead of the recording method using the near-field light by an SIL, on the other hand, there have been investigations for increasing recording density in the well-known optical recording method by irradiating an optical recording medium with a minute light spot.
As major features of optical disks, it is a removable medium, and a distributable ROM can be provided thereby. After the development of a DVD (digital versatile disk), a so-called DVR (digital video recorder) using an objective lens having an aperture of 0.85, for example, is developed to aim further high density recording, and in the case where a blue laser is used, a recording density of 20 GB (per disk of 12 cm in diameter) has been attained (Tech. Dig. of Optical Data Storage 2000, MB1, p. 15 (2000)).
As an example of the conventional optical head used for the DVR technique, an optical head disclosed in JP-A9-251662 can be exemplified.
The optical head has a light source generating a light wave, 2-group lenses formed with an objective lens for converging the light from the light source and a field lens arranged on the same axis as the optical axis of the objective lens, and a motor for moving the objective lens with respect to the objective lens in the direction along the optical axis. The aperture can be 0.85 by using the 2-group lenses, and a minute light spot can be formed. The spherical aberration can be relieved by changing the distance between the objective lens and the field lens.
However, the optical head using an SIL has such a problem that the height of the optical head is large because the objective lens 5 and the folding mirror 6 are arranged above the SIL 7.
As shown in FIG. 12, the optical axis of the objective lens 5 is conformed to the optical axis 7a of the SIL 7, and the optical axis 7a of the SIL 7 is arranged perpendicular to the optical disk 10. The configuration is customarily employed because the optical axis of the objective lens has been arranged perpendicularly to the surface of the optical disk in the conventional optical disk units, and the objective lens has been arranged perpendicularly to a sample in an oil immersion microscope (oil immersion lens), which is an origin of SIL.
The size of SIL itself has been decreased in recent years, and those having a size of 0.2 mm or less are being produced (S. Kittaka and Y. Sakai, Digest of Int. Symp. on Optical Memory 2000, Fr-J-15, p. 136 (2000)). However, the size of the objective lens is difficult to decreased to 1 mm or less, and therefore, the diameter of the laser beam is also of the similar size. Accordingly, the height of the optical head becomes 2 mm or more, which brings about a considerably large height of the head in comparison to the magnetic heads. Consequently, even when the surface recording density of the disk is increased to the similar level as the magnetic disks, the volume recording density is considerably lower than the magnetic disk units.
The similar problem occurs in the case of the optical system where a light spot is formed on the optical medium with the 2-group lenses.